A propeller of suitable design, when freely suspended for rotation about a vertical axis, can be operated by rising air currents generated, for example, by a heat source under the propeller. Generation of sufficient force to rotate such a propeller is dependent both on the configuration and size of the propeller and on the strength of the rising air currents, which in itself is dependent on the temperature of the heat source in relation to the surrounding atmosphere. Thus, freely suspended propellers can be used as temperature indicators since, for example, rotation of a particular propeller suspended above a particular heat source, such as a domestic stove, will only commence when the rising air currents from the stove are of a particular strength dependent on the stove attaining a particular temperature. Accordingly, rotation of the propeller is indicative of the source having attained a particular temperature. The present invention utilizes this principle to provide an improved temperature indicating device.